Unfortunately, you're kinda out of luck.
Try opening a console and typing
mount /dev/hda2
or
mount /dev/hda3
etc. That might work, though if Knoppix didn't detect them it might not be able to read them. Linux doesn't have good support for the Windows XP file system (NTFS) and can only read from drives. I don't know how well it handles NTFS, period.